1. Field of the Invention
The invention relates to a travelling mechanism of a rail-movable lifting device. More particularly, the invention relates to a travelling mechanism of a rail-movable lifting device with a friction wheel that is pressed onto a friction surface of the rail by a force that is proportional to a load on the lifting device.
2. Description of the Related Art
It is generally known to use ceiling lifts for transporting handicapped or sick persons to keep the ground free and clear. These ceiling lifts run on, for example, hollow-profile-type rails mounted in a region of the ceiling. The ceiling lift is moved along the rails by a travelling mechanism, which has a drive unit and a support unit that are connected to each other by an articulated rod.
The drive unit has running surfaces for travelling wheels that roll on the rails. These travelling wheels rest on the rails and are rotary-mounted on a shared carrier. The driving element is a motor, whose driving forces are transmitted to the travelling mechanism by a friction wheel that rests on a friction surface of the rails. The running surfaces and the friction surface are usually arranged parallel to each other. The support unit also has travelling wheels that, like the travelling wheels of the drive unit, roll on the running surfaces of the rails and are rotary-mounted on a shared carrier. The carrier is connected to the travelling mechanism below the rails.
In this known travelling mechanism for ceiling lifts, the friction wheel rests on the friction surface while being subjected to a pressure force. The pressure force must be selected such that flawless driving characteristics are ensured even at maximum load. For example, the friction wheel must not spin even at maximum load. In travelling mechanisms of this type, the friction wheel may be subject to high wear. Furthermore, the elastic friction wheel, because it is subjected to a pressure force while resting on the friction surfaces, experiences deformation if the ceiling lift is not moved for long periods of time. This deformation may be associated initially with an irregular running of the carrier when the lift is put into operation again. Moreover, the travelling mechanism is often difficult to move by hand because of the pressure force of the elastic friction wheel against the rail.
A travelling mechanism having a drive unit and support unit formed integrally is described in German reference DE 1288269. In this reference, the friction wheel is pressed onto a friction surface using a preloaded spring.
Another German reference, DE 26 18 516 C2, discloses a trolley of a rail-movable lifting device with a driven travelling wheel that serves as a supporting and driving wheel. The driven travelling wheel rolls on a running surface, which extends in the longitudinal direction of a rail and serves as the friction surface for the driven travelling wheel. An auxiliary travelling wheel is provided in the form of a wheel that is rotatably-mounted at a distance from the distance travelling wheel, seen in the direction of travel, on an end of a pivotable lever. During normal operation of the trolley the auxiliary wheel does not roll on the running surface of the rail, but instead remains in a parked position. In the event of failure or malfunction of the drive, the supporting wheel is placed onto the running surface by activating the lever; specifically, the free lever end of the lever, which is embodied as a double lever, is pulled vertically downward. The free end of the double lever is pulled downward until the auxiliary wheel assumes its working position, whereby the axis of the auxiliary travelling wheel is offset from the vertical, below the pivot axis, in the direction of the driven travelling wheel. In its working position, the auxiliary wheel interacts with a supporting wheel that is located across from the driven travelling wheel, seen in the direction of travel, such that the auxiliary wheel and the supporting wheel roll on the running surface of the rail, while the driven travelling wheel is lifted from the running surface Thereby enabling manual movement of the trolley even during malfunctions of the drive.
Finally, U.S. Pat. No. 1,367,522 discloses a travelling mechanism, which, however, has no force-pressurized friction wheel resting on a running surface of a rail. In this case, the drive is provided by a toothed wheel which engages, for example, into a toothed rack of the rail. This drive by means of interengaged teeth requires no pressure force, as is required in the case of a friction wheel that rests on a friction surface to drive the travelling mechanism.